Electrical protective circuit

ABSTRACT

A protective circuit for an isolated load circuit utilizing an isolation transformer having a primary winding connected to an associated source of power and a secondary winding connected to a load by a pair of load conductors which includes fault condition responsive means, a normally inoperative control circuit having a normally open electrical valve and a relay connected to the source of electrical power by an auxiliary transformer with an auxiliary circuit connected to the control circuit and to the load conductors for closing the valve at a predetermined voltage to operatively condition the control circuit upon the presence of a ground fault on one of the load conductors to energize the relay and actuate the fault condition responsive means.

United States Patent 1 Rogers, Sr. et al.

, [451 May 23, 1972 [54] ELECTRICAL PROTECTIVE CIRCUIT [72] Inventors:Charles J. Rogers, Sr., 1312 Coosaw Drive, Charleston, S.C. 29411; PaulE. Stanley, 100 l-lideway Lane, West I Lafayette, lnd. 47906 [22] Filed:July 17, 1970 [21] Appl. No.: 55,788

Primary Examiner-James D. Trammell AIt0mey-T. Russell Foster [57]ABSTRACT A protective circuit for an isolated load circuit utilizing anisolation transformer having a primary winding connected to anassociated source of power and a secondary winding connected to a loadby a pair of load conductors which includes fault condition responsivemeans, a nonnally inoperative control circuit having a normally openelectrical valve and a relay connected to the source of electrical powerby an auxiliary transformer with an auxiliary circuit connected to thecontrol circuit and to the load conductors for closing the valve at apredetermined voltage to operatively condition the control circuit uponthe presence of a ground fault on one of the load conductors to energizethe relay and actuate the fault condition responsive means.

2 Claims, 2 Drawing Figures Patented May 23, 1972 3,665,252

J RINVENTORJ 5 I ("5, Jr. BY PM a 52 11 ATTORNEY ELECTRICAL PROTECTIVECIRCUIT This invention relates to an electrical safety circuit and moreparticularly to such a circuit for providing ground fault protection inan electrical distribution system.

Many types of circuits have been proposed for providing protectionagainst personal injury and property damage in electrical powerdistribution systems such as are commonly found in residentialconstruction, industrial plants and the like; One type of such safetycircuits by means which outstanding results have been obtained inpreventing personal injury and property damage in an electricaldistribution system is the circuit described and claimed in U.S. Pat.No. 3,242,382 issued to C. .I. Rogers Sr.

Although the circuit of the aforementioned patent provides outstandingprotection certain areas, for instance, hospitals, where the use ofelectrical devices in operating rooms and the like is assuming an everincreasing importance, a very small magnitude of fault current and/orvoltage must be detected to avoid personal injury. For instance, leakagecurrent as low as microamps or even lower must be detected to insureabsolute safety. Even with an extremely low leakage current, thesensitivity of a patient undergoing an operation such as a heartoperation which requires the use of many electrical devices availabletoday, is such that electrical hazards are always present endangeringthe patients life. In addition, the detection of such a very low faultcurrent with ground fault interrupters in use today including thecircuit of the aforementioned patent is very difficult and has not beenaccomplished with the desired degree of success. Furthermore,interrupting the load circuit to such electrical devices upon thedetection of a ground fault current is not practical for medical use inthat the continuous functioning of the electrical devices in hospitalsand the like is required although the presence of a ground fault shouldbe known immediately so that corrective measure can be taken as soon aspractical.

Accordingly, a primary object of this invention is to provide a new andnovel electrical protective circuit for a power distribution system.

Another object of this invention is to provide a new and novelelectrical protective circuit for signaling the presence of a groundfault on a power distribution system without disconnecting the load.

A further object of this invention is to provide a new and novelelectrical protective circuit for detecting the presence of a groundfault on a power distribution system which is completely isolated fromthe power circuit by transformers.

Still another object of this invention is to provide a new and novelelectrical protective circuit for a power distribution system whichdetects extremely low fault currents in the system and which may beactuated at any selected ground fault voltage over a wide range.

Still another object of this invention is to provide a new and novelelectrical protective circuit for a power distribution system which isextremely reliable and stable, which utilizes a minimum of inexpensivereadily available components and which may be adapted for eithersignaling or load circuit interruption upon the presence of a groundfault on the system.

This invention further contemplates the provision of a new and novelelectrical protective circuit which is particularly adapted for use inthe systems distributing power to electrically operated devices used inhospitals, particularly operating rooms to eliminate injury and evendeath to patients from fault currents as low as two microamperes butwhich permits continuous use of such power while signaling the presenceof a fault condition.

In general, the objects of this invention and related objects areaccomplished by providing an isolation transformer having a primarywinding connected by a pair of line conductors to an associated sourceof electrical power and a secondary winding connected by a pair of loadconductors to a load. The circuit includes fault condition responsivemeans for actuation by a normally inoperative control circuit and meansare provided for connecting the control circuit to said source ofelectrical power and to ground. An auxiliary circuit is provided whichis connected to the control circuit and to the load conductors so thatupon the presence of a ground fault on one of the load conductors, theauxiliary circuit provides a ground fault circuit for operativelyconditioning the control circuit for actuation of the fault conditionresponsive means.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic wiring diagram of an electrical protective circuitconstructed in accordance with the invention; and

FIG. 2 is a schematic wiring diagram of a modification of the circuit ofFIG. 1.

Referring now to FIG. 1 there is shown one embodiment of the protectivecircuit of the invention which is utilized in connection with a powerdistribution system for connecting a load to a source of electricalpower. The circuit of FIG. 1 includes an isolation transformerdesignated generally by the numeral l2-preferably provided with a shield13 of conventional type and having a primary winding 14 and a secondarywinding 16. The primary winding 14 is arranged to be connected to anassociated source of electrical power (not shown) by means of lineconductors 17, 18. The isolation transformer 12 is preferably of thetype having a 1:1 winding ratio so as to provide the same voltage acrossboth the primary and secondary windings 14, 16.

The transformer secondary winding 16 is arranged to be connected to aload (not shown) by means of load conductors 21, 22 connected as shownto the terminal slots 23, 24 of a wall receptacle 26 or, if desired, theconductors 21, 22 may be connected to a receptacle of the anesthetizinglocation type such as described in NFPA Standard No. 56, FIG. A 2438. Inthe embodiment of FIG. 1, the receptacle 26 is of conventionalconstruction having, in addition to the slots 23, 24, a ground plug slot27 so that the slots 23, 24 and 27 are arranged to receive the threeprongs of a conventional plug by means of which a load such as anelectrical instrument, device or the like is connected to the loadconductors 21, 22.

The circuit of the invention includes fault condition responsive meanswhich in the embodiment of FIG. 1 comprise signaling means designatedgenerally by the numeral 30 for signaling the presence of a ground faulton one of the load conductors 21, 22. The circuit of FIG. 1 includes anormally inoperative control circuit for actuating the fault conditionresponsive means or signaling means 30. More specifically, the controlcircuit designated generally by the numeral 31, includes a relay 32having a coil 33 operatively associated with the signaling means 30. Therelay coil 33 is provided with terminals 33a, 33b and one side of therelay coil or terminal 33a is connected by means of conductor 34 to oneside of a normally open electrical valve 36 for maintaining the controlcircuit 31 in a normally inoperative condition. In the illustratedembodiment, the electrical valve 36 is preferably a silicon controlledrectifier or SCR having a gate 36a.

Means are provided for connecting the control circuit to the associatedsource of electrical power and to ground. More specifically, anauxiliary transformer designated generally by the numeral 38 is providedhaving a primary winding 39 connected by means of conductors 41, 42 toline conductors 17, 18 which, as described above, are connected to theassociated source of electrical power. The auxiliary transformer 38 hasa secondary winding 43 and means are provided for connecting thesecondary winding 43 to the control circuit 31 or, more specifically, tothe other side or terminal 33b of the relay coil 33 and to the otherside or gate 36a of the electrical valve or SCR 36.

More specifically, a pair of conductors 44, 46 are connected to the endsof the secondary winding 43 and a first pair of reversely positionedhalf-wave rectifiers or diodes 47, 48 are connected at their oppositesides to the conductors 44, 46 respectively. The corresponding sides ofthe diodes 47, 48 are connected together and to therelay coil terminal33b by means of conductor 49.

A second pair of half-wave rectifiers or diodes 51, 52 are providedwhich are reversely positioned oppositely from the position of the firstpair of rectifiers 47, 48 and have their opposite sides connected to theconductors 44, 46. The corresponding sides of the rectifiers 51, 52 areconnected together and to the SCR gate 36a by means of conductors 53,54.

Means are provided for grounding the control circuit 31 which in theembodiment in FIG. 1 includes a grounding con ductor 55 connected toconductor 53 and to the grounding slot 27 on the receptacle 26 as shown.The protective circuit of the invention also includes an auxiliarycircuit, designated generally by the numeral 60, connected to thecontrol circuit 31 and to the load conductors 21, 22. More specifically,the auxiliary circuit 60 includes a first pair of reversely positionedhalf-wave rectifiers or diodes 56, 57 which are connected at theiropposite sides to the load conductors 21, 22 respectively thecorresponding sides of the diodes 56, 57 being connected together bymeans of conductor 58. A second pair of halfwave rectifiers or diodes61, 62, reversely positioned oppositely from the position of the firstpair of rectifiers 56, 57 are provided in the auxiliary circuit 60 andhave their opposite sides connected to the load conductors 21, 22respectively. The corresponding sides of the diodes 61, 62 are connectedtogether by means of conductor 63 and to corresponding sides of thefirst pair of diodes 56, 57 by means of conductor 64.

The corresponding sides of both pair of diodes 56, 57 and 61, 62 areconnected to the SCR gate 36a by means of conductor 66 connected asshown to conductor 64. Resistances 67, 68 are preferably provided inconductors 64, 68 respectively for limiting the current flowtherethrough as will be explained hereinafter.

Means areprovided in the auxiliary circuit 60 for limiting the voltagein a ground fault circuit to a predetermined magnitude. Morespecifically, a zener diode 69 is provided in conductor 64 so that thezener diode is-connected at one side to the corresponding sides of thepair of diodes 61, 62 by means of conductor 63. The other side of thezener diode 69 is connected to the corresponding sides of diodes 56, 57and the SCR gate 36a by means of conductors 64, 66 respectively.

The signaling means 30 which is preferably electrically operated,includes an audible signal 71 such as a bell and a visual signal 72 suchas a light connected in parallel as shown. One side of the signals 71,72 is connected by means of conductor 73 to the one side or terminal 33aof the relay coil 33. The other side of the signals 71, 72 is arrangedto be connected to the auxiliary transformer secondary winding 43 bymeans of a conductor 74 and a switch 75 operatively as sociated with therelay 32 and arranged to interconnect conductors 53, 54. The switch 75is preferably of the double pole, double throw (SPT) type and isnormally maintained in the solid line position of FIG. 1 for normallydisconnecting the other side of the signals 71, 72 from the secondarywinding 43 and for normally interconnecting the conductors 53, 54 toconnect the SCR gate 36a to the secondary winding 43. The switch 75 ismovable by the relay 32 into the dotted line position of FIG. 1 forconnecting together conductors 53, 74 to connect the signals 71, 72 tothe secondary winding 43. Preferably, a resistor 76 is connected bymeans of a conductor 77 to conductors 66, 54.

Referring now to FIG. 2 there is shown a modification of the circuit ofFIG. 1 wherein like numerals have been used to identify like parts. Inthe circuit of FIG. 2, the signaling means 30 of FIG. 1 have beeneliminated and normally closed switching means have been provided fordisconnecting the load from the source of power upon the operativeconditioning of the control circuit. More specifically, the normallyclosed switching means, designated generally by the numeral 81, includesa pair of gang operated switches 82, 83 by means of which loadconductors 21, 22 are connected to the secondary winding 16 of thetransformer 12. The switching means 81 is operatively associated withthe relay 32.so that the switches 82, 83 are maintained in the normallyclosed position of FIG. 2.

The auxiliary transformer secondary winding 43 is connected to the SCRgate 36a by a conductor 86 connected by means of a conductor 87 to thecorresponding sides of the diodes 51, 52 and the control circuit 31 isgrounded by a grounding conductor 88 connected at one end to conductors86, 87 and at its other end to the ground conductor 18 connected to theprimary winding 14 of transformer 12 and to the associated source ofelectrical power.

In the operation of the circuit of FIG. 1, the presence of a groundfault on one of the load conductors 21, 22 is identified by the lettersGF and shown in broken lines between the load conductor 22 and thegrounding conductor 55. Such a ground fault may be in the form of arelatively small leakage current. As a result of the presence of theground fault GF, a ground fault circuit is established in the auxiliarycircuit 60 so that current flows through load conductor 22, through thesecondary winding 16, conductor 21, diode 56, conductors 58, 64 and 66,the SCR gate 36a, conductor 54, switch 75, conductor 53 the groundingconductor 55 and through the ground fault GF back to load conductor 22.The zener diode 69 clips the voltage in this ground fault circuit to thevoltage level as established by the rating of the zener diode. Forinstance, a 6 volt rated zener diode limits the voltage in the groundfault circuit to 6 volts.

When the current in the ground fault circuit reaches a predeterminedlevel, in accordance with the rating of the SCR 36, the value ofresister 76 and the various other resistors in the ground fault circuitthe current applied to the SCR gate 36a fires the SCR so as to permitcurrent to flow in the control circuit, power being obtained from thesecondary winding 43 of the auxiliary transformer 38. By way of example,current flows in the control circuit 31 from the right hand side of thesecondary winding 43 through conductor 46, diode 48, con ductor 49,relay coil 33, SCR 36, conductor 54, switch 75, conductor 53, diode 51conductor 44, and through the secondary winding 43 from the other side.The relay coil 33 is then energized to actuate switch moving it from thesolid line position to the dotted line connecting the signals 71, 72across the secondary winding 43 so that audible and visual signals areobtained indicating the presence of the ground fault GF on the conductor22. In the same manner, a ground fault on load conductor 21 results in asimilar operation of the circuit and an energization of the signalingmeans 30.

In the embodiment of FIG. 2, a ground fault on load conductor 22designated also by CF in FIG. 2, produces energization of relay coil 33for actuating the switching means 81 so that the switches 82, 83 move tothe open position disconnecting the load conductors 21, 22 from thesource of power. In the circuit of FIG. 2, the ground fault circuit iscompleted through conductor 88 connected as described above to thegrounded line conductor 18.

While there has been described what at present is considered to be thepreferred embodiment of the invention, it will be understood by thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention.

Having thus described the invention what is claimed is:

1. An electrical protective circuit for a power distribution systemcomprising, in combination, a transformer having a primary winding and asecondary winding, a pair of line conductors for connecting said primaryto an associated source of electrical power, a pair of load conductorsfor connecting said secondary winding to an electrical load, anauxiliary circuit connected to said load conductors, a normallyinoperative control circuit connected to saidauxiliary circuit, meansincluding an auxiliary transformer for connecting said control circuitto said line conductors, fault condition responsive means connected tosaid control circuit, means for grounding said control circuit, saidauxiliary circuit being arranged to provide a ground fault circuit uponthe presence of a ground fault on one of said load conductors foroperatively conditioning said control circuit for actuation of saidfault condition responsive means, and means in said auxiliary circuitfor limiting the voltage in said ground fault circuit to a predeterminedmagnitude, a silicon controlled rectifier having a gate in said controlcircuit in said normally inoperative condition and wherein said gate isconnected to said auxiliary circuit for actuation of said controlcircuit at a predetermined magnitude of fault current, said auxiliarycircuit comprising a first pair of reversely positioned rectifiershaving corresponding sides connected together and each connected attheir opposite sides to one of said load conductors, a second pair ofrectifiers reversely positioned oppositely from the position of saidfirst pair of rectifiers and having corresponding sides connectedtogether and each connected at their opposite sides to one of said loadconductors, and wherein said means for limiting the voltage in saidground fault circuit comprises a zener diode connected at one side tothe corresponding sides of said second pair of reversely positionedrectifiers and means for connecting the other side of said zener diodeto the corresponding sides of said first pair of reversely positionedrectifiers and to said silicon controlled rectifier gate.

2. An electrical protective circuit in accordance with claim 1 whereinsaid control circuit includes a relay having a coil operativelyassociated with said fault condition responsive means and wherein saidfault condition responsive means comprises an electrically operatiedsignal having one side connected to one side of said relay coil, meansincluding a switch operatively associated with said relay for connectingthe other side of said signal to said auxiliary transformer, means forconnecting said switch to said silicon controlled rectifier, said switchbeing normally maintained in one position for disconnecting said signalthe other side from said auxiliary transformer and for connecting theother side of said silicon controlled rectifier to said auxiliarytransformer, said switch being movable by said relay into a secondposition upon the operative conditioning of said control circuit forinterrupting said auxiliary circuit and for connecting said signal tosaid auxiliary transformer.

1. An electrical protective circuit for a power distribution systemcomprising, in combination, a transformer having a primary winding and asecondary winding, a pair of line conductors for connecting said primaryto an associated source of electrical power, a pair of load conductorsfor connecting said secondary winding to an electrical load, anauxiliary circuit connected to said load conductors, a normallyinoperative control circuit connected to said auxiliary circuit, meansincluding an auxiliary transformer for connecting said control circuitto said line conductors, fault condition responsive means connected tosaid control circuit, means for grounding said control circuit, saidauxiliary circuit being arranged to provide a ground fault circuit uponthe presence of a ground fault on one of said load conductors foroperatively conditioning said control circuit for actuation of saidfault condition responsive means, and means in said auxiliary circuitfor limiting the voltage in said ground fault circuit to a predeterminedmagnitude, a silicon controlled rectifier having a gate in said controlcircuit in said normally inoperative condition and wherein said gate isconnected to said auxiliary circuit for actuation of said controlcircuit at a predetermined magnitude of fault current, said auxiliarycircuit comprising a first pair of reversely positioned rectifiershaving corresponding sides connected together and each connected attheir opposite sides to one of said load conductors, a second pair ofrectifiers reversely positioned oppositely from the position of saidfirst pair of rectifiers and having corresponding sides connectedtogether and each connected at their opposite sides to one of said loadconductors, and wherein said means for limiting the voltage in saidground fault circuit comprises a zener diode connected at one side tothe corresponding sides of said second pair of reversely positionedrectifiers and means for connecting the other side of said zener diodeto the corresponding sides of said first pair of reversely positionedrectifiers and to said silicon controlled rectifier gate.
 2. Anelectrical protective circuit in accordance with claim 1 wherein saidcontrol circuit includes a relay having a coil operatively associatedwith said fault condition responsive means anD wherein said faultcondition responsive means comprises an electrically operatied signalhaving one side connected to one side of said relay coil, meansincluding a switch operatively associated with said relay for connectingthe other side of said signal to said auxiliary transformer, means forconnecting said switch to said silicon controlled rectifier, said switchbeing normally maintained in one position for disconnecting said signalthe other side from said auxiliary transformer and for connecting theother side of said silicon controlled rectifier to said auxiliarytransformer, said switch being movable by said relay into a secondposition upon the operative conditioning of said control circuit forinterrupting said auxiliary circuit and for connecting said signal tosaid auxiliary transformer.